The present invention relates generally to shirred cellulosic food casing articles and to apparatus and manufacturing methods. In particular, the invention relates to such an article compacted to form a shirred casing stick which exhibits a more uniform pack ratio over the length of the article than shirred casing sticks compacted to the same length by prior art methods.
The invention also relates, in particular, to a compaction method for achieving a relatively high pack ratio over the length of the article, for reducing casing damage, and for increasing the pack ratio obtainable for a given compaction force.
Shirred tubular casings are well known in the art. Such casings are extensively used in food processing to make a variety of sausage type products and in the packaging of larger food products such as cooked and smoked ham and the like.
Casings are of several different types and sizes to accommodate the different categories of food products to be prepared. Moreover, casings can be reinforced as needed, such as by embedding a fibrous support web in the casing wall for added strength.
Casings employed in the preparation of small size sausage products, such as frankfurters, are generally designated "small food casings". As the name suggests, this type of food casing is small in stuffed diameter. Generally, it has an inflated diameter within the range of from about 13 mm to about 40 mm. Small food casings are most usually supplied as thin-walled tubes of very great length. For convenience in handling and use, lengths of these casings are shirred and compressed to produce what is commonly referred to in the art as "shirred casing sticks". A shirred casing stick is an integral, self-supporting article from about 20 cm to about 60 cm in length which contains from 20 to about 50 meters (2000 to 5000 cm) of casing. Shirring machines and shirred casing sticks are shown in U.S. Pat. Nos. 2,983,949 and 2,984,574, among others.
"Large size food casing", is the common designation for casings used in the preparation of generally larger food products, such as salami and bologna sausages, meat loaves, cooked and smoked ham butts, and the like. The stuffed diameter of large size food casings range from about 40 mm to about 200 mm or even larger. In general, large size food casings have a wall thickness about three times greater than the wall thickness of "small size casings" and are provided with a fibrous web reinforcement embedded in the wall, although they may be prepared without such reinforcement. Large size casings of both types have been, and are being, supplied in the form of shirred sticks containing up to 65 meters of casings for stuffing with high speed apparatus.
Shirring techniques for the casings described hereinabove, in accordance with patent references noted, as well as with others, can be generally described as involving the continuous feeding of a length of flat casing feed stock, from a roll for instance, onto a mandrel of a shirring machine. The flat casing is inflated with low pressure gas, usually air. Then the inflated casing is passed through an array of shirring rolls or other shirring means which pleat the casing up against a restraint, on or about the mandrel, until a preselected shirred length has been attained.
The pleats formed in conventional shirring operations are usually laid at an angle to the axis of the mandrel so that the pleats can be described as nesting one into the other, much like a stack of nesting conical elements. Shirring in cooperation with conventional hold back means compacts the nested pleats. After the shirring operation, a strand of the shirred casing is subjected to an axial compressive force which further compacts the shirred casing to form the shirred casing stick.
A shirred casing stick is generally a coherent, self-supporting article which is capable of handling by conventional automatic stuffing apparatus. The coherency or ability of the stick to maintain a structural and mechanical integrity during the rigors of packaging, storage and handling is thought to be the result of the nesting pleats being forced into a closer association, one to another, by the compacting process.
Shirring machines are generally of two types, a floating mandrel type and a withdrawing mandrel type. With a floating mandrel type of shirring machine, as described for example in U.S. Pat. No. 3,766,603, a strand of the shirred casing is transferred linearly, beyond or away from the restraint against which the shirring was performed, and onto an extended mandrel portion. In this new location, the strand of shirred casing is compacted by an axially applied force to produce a coherent, self-supporting shirred casing stick length, which may be about 1.0 percent to about 1.2 or 1.3 percent of the original, unshirred, casing length.
Similar results are achieved with a withdrawing mandrel shirring machine, as shown for example in U.S. Pat. No. 2,583,654; except that in this type of machine, the shirring mandrel, with the shirred casing thereon, is rotated or otherwise indexed to an alternative position where the shirred casing is compacted.
Compaction is described in the art in terms of "pack ratio" which is simply the ratio of the length of the unshirred casing to the length of the shirred, compacted casing stick. This ratio, generally, has been in the order of 70 to 100; that is 70 to 100 feet of casing being shirred and compacted to a stick measuring about one foot in length. One U.S. Pat. No. 2,001,461, speculates that the lowest practical limit of length reduction is in the neighborhood of 1/130th (i.e., a pack ratio of 130), but indicates a preferred length reduction of about 1/80th (i.e., pack ratio of 80).
It should be appreciated that sticks having high pack ratios are desired for optimum operation of a continuous, automatic stuffing machine. The greater the length of casing that is compacted into a relatively short stick, the greater the amount of stuffed product that can be made before the supply of casing is depleted.
While shirred casing sticks having high pack ratios are desired, as a practical matter there are a number of factors which tend to discourage manufacture of sticks having pack ratios much in excess of about 70 to 100.
For example, as the pack ratio increases, the likelihood of casing damage increases. This damage is manifested by pinholes in the casing. Pin holes are thought to be caused by the friction between adjacent pleats of the shirred casing as the compaction force pushes the pleats axially one against the other, and by friction between the pleats and the mandrel. As a general rule, there should be no more than 11/2 pinholes per 10,000 feet of casing.
Also, it is known that when a shirred casing stick is compacted, the diameter of its bore tends to decrease. This is believed to be caused by the individual pleats of the shirred casing being forced to assume a more upright orientation by the axially applied compacting force. This causes the pleats to expand or otherwise grow inwardly as the shirred casing is compacted axially. A decrease in the diameter of the stick bore is manifested by the tendency of the stick to seize about the mandrel during compaction.
It is also known that even after the compaction force is released and the stick is doffed from the mandrel, the bore will continue to grow smaller over a period of time. This inward growth of the pleats, both during the compaction process and, subsequently, after doffing, has been found to vary in direct proportion to the magnitude of the compaction force. The greater the force applied to increase pack ratio, the greater the likelihood of the pleats seizing about the mandrel and growing inward after doffing, both of which tend to occlude the bore of the stick.
Any reduction in bore diameter of the stick is in opposition to the desirable feature of having as large a bore diameter as possible. Maintaining as large a stick bore as possible is desirable because it will permit the stick to fit over, or otherwise accommodate, the largest stuffing horn in terms of cross sectional area for a given casing size. This is important because it is generally desirable to stuff at the lowest possible pressure. Maximizing the internal cross sectional area of the stuffing horn will maximize product throughput at minimum stuffing pressure.
Concurrent with bore reduction, it is also recognized that the compacted casing stick will begin to grow and elongate as soon as the compacting force is released. The greatest proportion of growth occurs immediately after the compacting force is released. Thereafter, the growth gradually diminishes. Accordingly, it is recognized in the art that a compacted shirred casing stick is resilient and has the potential of storing a portion of the energy exerted in compacting the shirred casing.
Stick growth will reduce peak ratio (casing length divided by stick length) so this requires the compacting operation to produce a stick which has a higher pack ratio than the desired, finished, or doffed stick pack ratio. Consequently, it may be necessary to subject the casing to the likelihood of pinhole damage from high compaction forces even though the pack ratio of the finished or doffed stick is below a pack ratio range causing such damage.
It has been found that the magnitude of the forces generated by both the inward growth of the pleats and stick expansion as set out above, are proportional to the longitudinal compacting force used to compress the stick to the compressed length. That is, the axial and radial inward forces generated by the compacted stick, increase as the longitudinal compacting force increases.
It also has been observed, particularly for the small size casings used to make frankfurter type products, that when a shirred stick is highly compacted in an effort to maximize its pack ratio, the coherency or structural integrity of the compressed stick deteriorates to a point whereby the stick is rendered nonfunctional. The stick becomes fragile, is easily broken, and cannot be automatically handled by a stuffing machine, nor mounted on a stuffing horn. Since the shirring process is known to produce pleats which nest one with another, much the same as a stack of nested cones, it is speculated that this loss of coherency when excessive compaction forces are applied, occurs because such forces tend to straighten out the "nesting cone" geometry produced during the shirring process. Thus, while compaction is needed to form a coherent stick, increasing the compaction force beyond some point will work to reduce coherency.
Prior to the present invention, compaction of a shirred casing was accomplished by any one of several single-ended compression methods accomplished with the shirred stick on a mandrel.
In single-ended compression, a first end of the shirred casing is held against a restraint while a movable compaction arm applies an axial force to a free second end of the casing. After compaction in this manner, the casing is doffed from the mandrel.
In another variation, the force is applied first to one end and then to the other. In this case, the strand of shirred casing first undergoes a single-ended compression as set out above. Then, the restraint is released and shifted to hold the second end of the casing while an axial force is applied to the first end. This can be accomplished by doffing the shirred casing stick in order to turn it end for end on the mandrel, by turning the mandrel end for end with the casing stick still mounted thereon, or by opening the restraint to pass the shirred casing along the mandrel, and then closing the restraint and applying an axial force in the reverse direction.
In the latter variation, the force is applied first to the one end and then to the other, so the term single-ended "sequential compression" would be an apt description of the method. Compaction methods as set out above are more particularly described in U.S. Pat. Nos. 2,001,461, 3,209,398 and 3,112,517.
"Compaction", as used herein, should be understood to mean compaction while controlling the inside diameter or bore size of the shirred casing. This requires compaction about a member which establishes the desired inside diameter of the shirred casing during compaction to limit inward growth of the casing pleats. Prior art patents as discussed below, while silent on the aspect of controlling bore size, are assumed to provide such control, and therefore, are subject to the generation of frictional forces between the casing and the mandrel on which the casing is compacted.
Hewitt, U.S. Pat. No. 2,001,461 and Ziolko, U.S. Pat. No. 3,209,398, show versions of single-ended compression. In Hewitt, the shirred casing is dropped over a plunger (mandrel) wherein upward movement of the plunger through an abutment compresses the shirred casing against the abutment. According to Hewitt, the highest practical limit of pack ratio probably is in the neighborhood of 130, while a preferred range is about 80. Ziolko, likewise shows moving a mandrel through a casing restraint or abutment. Ziolko, in addition, moves the casing through successive compression stations to apply an increasing axial force to the shirred casing.
Ives, U.S. Pat. No. 3,112,517 shows a sequential compression wherein the casing is transferred along a mandrel for compression, first in one direction, and then in the other.
Compacting a shirred casing on a mandrel, as in both single-ended and single-ended sequential methods, does, to some extent, control bore size, because the pleats cannot expand inward beyond the outside diameter of the mandrel. However, when applying the relatively high compaction forces needed to obtain doffed pack ratios greater than about 70, the inwardly growing pleats frictionally engage about the mandrel. This frictional engagement adds to the likelihood of the stick seizing on the mandrel and to the likelihood of pinholes or other casing damage.
Coating the mandrel with a friction reducing material, is known in the art and does not completely alleviate the problem. As an alternative, one could use a mandrel having a small diameter compared to the bore size of the shirred casing. While this would eliminate, or considerably reduce, friction between the mandrel and casing during compaction, it also has the negative effect of allowing the pleats freedom to grow inwardly, thereby losing control over the bore size of the shirred casing stick.
Even discounting damage to the casing when using a mandrel to control bore size, friction between the casing and mandrel is a limiting factor to obtaining high pack ratios, for still another reason. In this respect, greater and greater forces must be applied to overcome the increasing friction between the pleats and mandrel as the pleats grow inward and seize about the mandrel. At some point, a limit is reached wherein pinhole damage produced by the increasing compaction forces exceed acceptable limits.
Close examination of shirred casing sticks made according to the prior art methods of single-ended and single-ended sequential compaction, has disclosed a further pack ratio characteristic of these methods. For sticks made with single-ended compression, the pack ratio gradually diminishes from one end of the stick to the other. In its compacted condition, the restrained end has the lower pack ratio, and the end nearest to the compacting arm, has the higher pack ratio. In sticks made with single-ended sequential compression, the pack ratio tends to be higher at each end and generally lower around the midpoint of the stick.
It is believed that friction between the pleats and the mandrel, as the pleats move along the mandrel, is the cause of this uneven pack ratio distribution. The end of the stick adjacent the compaction arm has its pleats compacting and growing inward to engage about the mandrel sooner than the pleats at the restrained end of the stick. Consequently, the resistance to compaction at the compaction arm increases as more and more pleats begin to frictionally engage the mandrel. In an extreme case, the end of the shirred casing farthest from the compaction arm would experience little or no compaction, because the applied compaction force is effectively resisted, or balanced, by the friction of the pleats towards the compaction arm. This theory seems to be substantiated by the pack ratio distribution as exhibited in sticks formed by single-ended sequential compression as set out hereinabove.
Casing length is another factor in the uniformity of the pack ratio distribution. The distribution is more uniform for shorter lengths of casing. As casing lengths increase, the uniformity of pack ratio tends to change over the length of the stick.
Maintaining a pack ratio as close as possible to the maximum (short of pinhole damage) over the full length of the stick translates to more casing per given stick length and a higher overall or average pack ratio. Having a near maximum pack ratio at one end of the stick, or the other, and a lower pack ratio at another part, translates to less casing per given stick length and a lower overall or "average" pack ratio.
In the production of shirred casing sticks, a goal in the art has been to maximize the amount of casing contained in a stick having a given bore diameter. This goal can be quantified by the packing efficiency (PE) of the stick.
Simply put, PE is a ratio indicative of the density of a shirred casing stick. It is computed by calculating the actual volume of casing material contained in a given shirred and compacted stick length, and then dividing by the volume of a solid object having the same dimensions of the shirred stick (ie. length, inside diameter and outside diameter).
From this relationship, it is seen that the maximum packing efficiency for any shirred stick is unity. Increasing the pack ratio works to increase the density of the stick and, therefore, the packing efficiency. Thus, the desire to maximize packing efficiency is another reason for wanting to increase the pack ratio to the maximum value short of pin hole damage and to obtain as constant a pack ratio over the full length of the stick as possible.
In practice, conventional shirred casing sticks have had characteristics representing a compromise from the characteristics of an "ideal" stick in order to balance the competing factors of: little or no growth in terms both of length and of reduction in bore size after removal of the compaction force; high pack ratio to maximize packing efficiency; coherency; and large inner diameter or bore size.
For example, all things being equal, a higher pack ratio can be produced by downsizing the mandrel to reduce friction, but, reducing mandrel size sacrifices control over bore size. Upsizing the mandrel to control bore size increases friction and sacrifices pack ratio.
In either case, applying more force to increase the pack ratio risks an increase in pinholing and increases the likelihood of bore reduction and stick growth after doffing.
In a copending application, Ser. No. 363,851 filed Apr. 5, 1982 in the name of Mahoney, et al, a shirred casing article, termed a "cored high density" or CHD article, is disclosed. A CHD article has a food casing shirred and highly compacted on a tubular core. As described in said patent application, acceptable CHD casing articles having higher than conventional packing efficiencies and pack ratios were obtained when the casing was shirred and compacted on a tubular core.
Use of a tubular core, which takes up space in the bore of the shirred casing, was expected to decrease the bore size of the finished article. Contrary to what was expected, it was found that by compacting about a core it was possible to compact longer than conventional casing lengths to higher than conventional pack ratios without a significant reduction in bore size.
Provided the core is sufficiently rigid to resist the axial and radial forces generated by the compacted casing, it was unexpectedly found that the space taken up by the core did not reduce the bore size. The core resisted inward growth of the casing, so that much higher forces could be applied to attain the higher pack ratios without the previously experienced consequence of bore reduction.
Other surprising results, and the advantages of compacting to a high pack ratio about a core, are more fully set out in said patent application. It is sufficient for purposes of the present invention merely to say that the data obtained from the examples, as set out in said patent application, clearly establishes the benefits of using a rigid tubular core to resist inward growth of shirred casing compacted to higher than conventional pack ratios. The data from said examples also demonstrates and confirms that elongation or growth in stick length occurs after the compacting forces have been released.
When producing a CHD article, wherein longer than conventional lengths of casing (up to 200 feet or more) are shirred and routinely compacted to higher than conventional pack ratios (above 100) about a tubular core, the disadvantages of single-ended and single-ended sequential compaction techniques becomes most evident.
In both techniques, the forces generated by the compacting casing contribute to core failure. In one aspect, cores were found to undergo a localized buckling compression failure during compaction as more and more of the axial compaction loading was transmitted to the core by the compacting casing. Cores which did not fail in this fashion sometimes collapsed or underwent a reduction in bore size, when doffed, due to the loading caused by radial inward expansion of the compacted casing.
The present invention seeks to provide a compaction method which permits an improvement in the desirable characteristics of a shirred casing stick. In particular, the compaction method of the present invention uses less force than prior art compaction methods to produce a given pack ratio. The present invention also is able to produce a stick having a more uniform pack ratio distribution over the length of the stick for a give compaction force than prior art compaction methods. The invention further permits an increase in the pack ratio over that which can be obtained with the same force applied in prior at compaction techniques, obtains higher pack ratios without increasing pinhole damage, and increases pack ratios while reducing the likelihood of a commensurate increase in stick growth or reduction in bore size.
When used in connection with CHD articles, as set out in the above mentioned copending patent application, the present invention provides a compaction method which reduces the tendency of the core to buckle or to collapse under the radially inward forces exerted by the casing on the core or to elongate after doffing.
With respect to the present invention, it has been found that the desirable attributes of a shirred casing stick, such as a greater pack ratio uniformity, low stick growth, maintenance of bore size after doffing from the compaction mandrel, as well as certain other advantages, can be achieved by a double-ended simultaneous compaction method.
In a double-ended simultaneous compaction method according to the present invention, both ends of the shirred casing are displaceable and are free to move with respect to the size controlling mandrel, each end toward the other responsive to forces applied simultaneously to both ends of the casing. It is believed that this method results in less friction between the casing and mandrel, and less opportunity for friction induced casing damage, so that greater, more uniform pack ratios can be achieved.
Shirred casings, compressed in this manner, have also evidenced less pinhole damage at higher pack ratios and have shown a reduced tendency to elongate or to undergo a reduction in bore size after doffing from the compaction mandrel.
Compaction to a given pack ratio according to a double-ended simultaneous method as described herein, is accomplished with less applied force than other prior art compaction techniques. Less applied force during compaction means there is less stored energy available for subsequent release by the compacted shirred casing stick in the form of stick growth and inward expansion of the individual pleats.
Elongation and reduction in bore size are further reduced in the case of casing articles using a core member as set out in the copending application mentioned above. This is due in part to the friction between the core and casing and the rigidity of the core material.
The compaction method of the present invention when used to compact a cored, shirred casing article, also affords the opportunity to capture the fully compacted length of casing on the core, as more fully described in a copending application, Ser. No. 434,559, filed Oct. 15, 1982, now U.S. Pat. No. 4,493,130.
It is possible to practice the method of the present invention on either a floating or withdrawing mandrel type of shirring machine, in several ways. For example, in one way, the shirred casing can have its midpoint fixed with respect to the mandrel, while forces are simultaneously applied to each end of the casing for moving each end relative to the mandrel and toward the midpoint. These forces can be applied by locating the casing between two compactor arms and then simultaneously moving each arm towards the other against the opposite ends of the casing.
In another arrangement, the casing is placed on a size controlling sleeve which can slide along the mandrel. One end of the casing is placed against an end restraint while the sleeve is allowed to slide through the restraint. This permits both ends of the casing to displace each toward the other and with respect to the sliding sleeve as a compaction force is applied to the other end of the casing. It is believed that allowing the sleeve the freedom to slide with respect to the mandrel and through the restraint, has the same effect as compacting the casing between two compaction arms which are moved together for the application of substantially simultaneous, equal and opposite forces to the ends of the shirred casing located about the sleeve.